The transient creep of graphite in a reactor environment

In a reactor environment, graphite exhibits two creep mechanisms; transient creep and steady creep. The creep response is temperature dependent and is linear with respect to the applied stress. The present author, and others, have proposed a 4-parameter model consisting of Maxwell and Kelvin elements in series, the Maxwell element models the elastic and steady creep response and the Kelvin element models the transient creep. Stress analyses of the graphite core components of the High Temperature Reactor (HTR) have hitherto neglected the transient creep: the response of the graphite has therefore been modelled by a Maxwell element only. In this paper the author re-examines the influence of transient creep. The differential equation of the 4-parameter model is derived and integrated along assumed stress paths to obtain algorithms for computing the incremental strains. Relationships are obtained between the parameters of the model and the measured properties of graphite. The algorithms have been incorporated into computer programmes which analyse the stress, strain and deformation history in HTR core components of various geometric forms. The paper includes results of these analyses for several values of the total transient creep strain, embracing the maximum values indicated by recent experiments. The results are compared with the results which are obtained by neglecting the transient creep. On the basis of these comparisons, the author concludes that transient creep may have a significant effect on the stresses developed during periods in the life of the fuel elements when the heat rating is changing, as, for example, during load following operation. (Auth.)